Switchable rocker arm

ABSTRACT

A latching mechanism for variably activating an engine valve in an internal combustion engine includes a rocker arm body having a bore and a socket. A first passage leads from the socket to a first hydraulic chamber formed above a lock pin head. A second passage leads from the socket to a second hydraulic chamber formed below the lock pin head. Since both the first and the second hydraulic chamber receive pressurized oil, it is the diameter of the pin shank that determines the hydraulic force needed to axially move the locking pin to an extended position and into a locking engagement with a slider member. In an alternate embodiment of the invention the first passage connecting the first hydraulic chamber with the socket is eliminated and instead the locking pin is modified to permit flow between second hydraulic chamber and first hydraulic chamber.

TECHNICAL FIELD

The present invention relates to mechanisms for altering the actuation of valves in internal combustion engines; more particularly, to finger follower type rocker arm assemblies capable of changing between high and low or no valve lifts; and most particularly, to an improved oil circuit for a latching mechanism for a two-step finger follower type rocker arm assembly.

BACKGROUND OF THE INVENTION

Variable valve activation (VVA) mechanisms for internal combustion engines are well known. It is known to be desirable to lower the lift, or even to provide no lift at all, of one or more valves of a multiple-cylinder engine, during periods of light engine load. Such deactivation or cam profile switching can substantially improve fuel efficiency.

Various approaches are known in the prior art for changing the lift of valves in a running engine. One known approach is to provide a latching mechanism in the hydraulic lash adjuster (HLA) pivot end of a rocker arm cam follower, opposite from the valve-actuating end, which locks and unlocks the valve actuator portion from the follower body. The cam follower mechanism is latchable by a hydraulically actuated lock pin whose motion typically is governed in a latching direction by application of pressurized engine oil received from the HLA and in an unlatching direction by a return spring. The lock pin is disposed as a piston in a smooth bore of the follower body and is retained therein by a plug pressed into the end of the bore. The typically cylindrical plug serves to seal the prior art smooth bore, thus forming a hydraulic chamber between itself and an end of the lock pin.

Typically, a two-step roller finger follower (RFF) allows the engine valves to be operated with two different cam profiles, one when the locking pin disengages (unlocks) a high lift follower (low lift mode) and the other when the locking pin is engaged (locked) to the high lift follower (high lift mode). When the HLA oil pressure is low, the return spring moves the locking pin to a retracted position and the locking pin is disengaged from the high lift follower or other valve actuator. When HLA oil pressure is increased, the hydraulic force of the oil pressure in the hydraulic chamber overcomes the spring force and the locking pin moves to an extended position engaging with the high-lift follower or other valve actuator. However, this prior art oil circulation arrangement suffers from several shortcomings.

First, the major diameter of the lock pin, for example, the head diameter if the lock pin includes a head and a shank, determines the magnitude of the hydraulic force generated by the oil pressure in the hydraulic chamber, which may be determined as oil pressure times pin head area, and the total volume of fluid displaced as the pin moves. The lock pin response depends, therefore, on the capacity of the system to flow the required volume of oil. Problems may occur when moving the lock pin from an extended position to a retracted position with cold oil due to the high viscosity of the oil and, thus, a lower flow rate through the HLA, at lower temperatures and at relatively low pressure. A smaller major diameter of the lock pin would reduce the required flow capacity but reduction in size is constrained by the contact stress between the lock pin and the saddle of the high lift follower.

Second, the tolerances on the diameter of the lock pin head and surrounding bore must be controlled to manage the leakage through the clearance. Consequently, manufacturing costs are increased. The axial length of the lock pin head also influences the leakage. While a longer axial length of the lock pin head would reduce the leakage, a shorter length is beneficial for packaging both the return spring and the rocker arm in the cylinder head. However, tight clearances between the lock pin head and the bore combined with shorter pin head length increases the risk of jamming the lock pin head in the bore.

Third, the return spring cavity is typically vented such that clearance between the diameter of the pin shank (minor diameter) and bore is not required to seal against oil leakage. However, tolerances on the minor diameter size cannot be increased, as the diameter must be controlled for reasons of mechanical lash.

Fourth, a passage connecting the bore with the HLA intersects the bore of some prior art two-step finger follower type rocker arm assemblies at an angle, which may limit the hole size of the passage. The lock pin head and retention plug can in some applications partially block the passage and, thus, the flow area. Further, the angled hole may be difficult to drill and may be prone to burrs in critical areas.

What is needed in the art is an improved latching mechanism for a two-step finger follower type rocker arm assembly that more effectively moves the lock pin from a retracted position to an extended position, that eliminates the currently required tight clearances between lock pin head and bore, and that concurs with return spring and rocker arm packaging requirements.

It is a principal object of the present invention to provide a two-step rocker arm assembly having faster response characteristics under cold oil conditions among members of a manufactured population of two-step finger follower type rocker arm assemblies.

It is a further object of the invention to provide an improved oil circuit for a latching mechanism for a two-step finger follower type rocker arm assembly.

It is a still further object of the present invention to provide a lock pin mechanism that reduces the net volume of oil displaced by the lock pin and reduces the required return spring force for equivalent switch pressure.

SUMMARY OF THE INVENTION

Briefly described, a rocker arm body includes a first and a second oil passage connecting a lock pin bore with a variable oil pressure source, such as a socket for a hydraulic lash adjuster (HLA). The first oil passage leads from the HLA socket to a hydraulic chamber formed by a lock pin head and a retention plug sealing the lock pin bore. The second oil passage leads from the HLA socket to a return spring cavity. Adding the second oil passage will not increase the number of machining operations compared to the known prior art, since at the same time a venting hole typically machined into the follower body can be eliminated. Therefore, oil flow between the HLA and the return spring cavity and between the HLA and the hydraulic chamber above the lock pin head is enabled. Furthermore, leakage between the lock pin head and the wall of the lock pin head is advantageous rather than detrimental as in prior art. Due to the oil circuit in accordance with the invention, the minor diameter of the lock pin, which may be the diameter of the lock pin shaft, now determines the hydraulic force generated by oil pressure and the net volume of oil displaced by the lock pin motion. Compared to prior art lock pin mechanisms, up to 40% less oil needs to be displaced as the pin translates between the extended and retracted positions. Consequently, the viscous flow losses are reduced and switching response is improved, especially with cold oil. Since a smaller hydraulic force is needed to move the lock pin compared to prior art oil circulation, the required return spring force for equivalent switch pressure is reduced compared to prior art, which enables a superior return spring design, such as having a lower rate and/or shorter length.

Furthermore, with the introduction of the second oil passage that allows oil to flow to both sides of the lock pin head in accordance with the invention, oil leakage past the lock pin head is no longer a concern and, therefore, related tolerances can be relaxed compared to known prior art latching assemblies and the axial length of the pin head can be reduced. The shorter axial length of the lock pin head combined with larger allowable clearances between the lock pin head and the wall of the lock pin bore alleviates the lock pin head jamming concern of the prior art and reduces the chance of the lock pin head partially blocking the original passage where the passage breaks into the lock pin bore.

In an alternate embodiment in accordance with the invention, the first oil passage connecting the HLA socket with the hydraulic chamber above the lock pin head is completely eliminated and the lock pin is modified to permit oil flow between the return spring cavity and hydraulic chamber as well as from the HLA socket to and from the return spring cavity. Eliminating the original oil passage connecting the HLA socket with the hydraulic chamber is advantageous, since the typically angled flow passage is difficult to manufacture and may have potential for burrs in critical regions of the passage.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:

FIG. 1 is a cutaway isometric view of a prior art two-step roller finger follower;

FIG. 2 is a cross-sectional side view of a first latching mechanism of a two-step finger follower rocker arm assembly in accordance with the present invention, showing a first embodiment of an oil circuit; and

FIG. 3 is across-sectional side view of a second latching mechanism of a two-step finger follower rocker arm assembly in accordance with the present invention, showing an alternative embodiment of an oil circuit.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The advantages and benefits afforded to a two-step roller finger follower (RFF) in accordance with the invention may be better appreciated by first considering a prior art two-step roller finger follower. Such a two-step RFF is suitable for use in a variable valve activation system of an internal combustion engine.

Referring to FIG. 1, the view shown represents a section cutaway along a vertical symmetry plane for description purpose such that only one-half of a prior art two-step roller finger follower (RFF) 10 is present. Thus, where appropriate, the described elements should be considered as having matching counterparts (not shown) in the full RFF. A high-lift follower 12 including a cam-follower surface 14 is disposed in a central opening 16 in a rocker arm 18. High-lift follower 12 pivots within opening 16 about a pivot shaft 20. A roller shaft 22 mounted in rocker arm 18 supports a roller 24 for following a low-lift lobe of an engine camshaft (not shown).

Rocker arm 18 includes a body 26 having a first end 28 and a second end 30. A socket 32 is included at first end 28 for pivotably mounting RFF 10 on a hydraulic lash adjuster (HLA) (not shown). A pad 34 is included at second end 30 for actuating a valve stem (not shown). A latching mechanism 40 disposed in body 26 of rocker arm 18 at the first end 28 thereof selectively latches high-lift follower 12 in position to actuate the valve stem in response to the high-lift cam lobe base circle and eccentric, or selectively unlatches high-lift follower 12 to follow the high-lift cam lobe base circle and eccentric in lost motion.

Latching mechanism 40 includes a stepped bore 42, preferably cylindrical, in body 26. Latching mechanism further includes a piston 44 defining a locking pin having a head portion 46 and a shank portion 48. Head portion 46 of piston 44 may be biased outwards in bore 42 by a return spring 50 (retracted position) or shank portion 48 may be extend toward high-lift follower 12. Bore 42 is closed by a plug 52 forming a hydraulic chamber 54 that is in communication via passage 56 with socket 32.

Pressurized oil is supplied to chamber 54 in known fashion from the hydraulic lash adjuster (not shown) upon command from an engine control module to cause piston 44 to become hydraulically biased toward high-lift follower 12. The diameter of head portion 46 of piston 44 determines the magnitude of the hydraulic force (oil pressure times area of head portion 46) available to overcome the spring force and also the total volume of fluid displaced as the piston 44 moves. When such biasing occurs and overcomes the counter-bias of return spring 50, shank portion 48 extends from bore 42 towards high-lift follower 12 and locks high-lift follower 12 in position. Oil leaking past head portion 46 of piston 44 is vented from return spring cavity 58 via a vent hole (not shown) included in body 26 in the cutaway section.

When the engine control module determines, in known fashion from various engine operating parameters, that high-lift follower 12 should be unlocked, a reduced oil pressure is supplied to chamber 54, allowing return spring 50 to bias piston 44 away from high-lift follower 12, and high-lift follower 12 is again free to pivotally slide in central opening 16. As long as the lower oil pressure is supplied to chamber 54, latching mechanism 40 remains disengaged from high-lift follower 12. To minimize oil leakage from hydraulic chamber 54 into a return spring cavity 58 past head portion 46 of piston 44, the clearance between the diameter of head portion 46 and bore 42 must be carefully controlled with appropriate tolerances. Leakage through the clearance between head portion 46 and bore 42 may be controlled by designing head portion 46 of piston 44 to have a longer axial length 45, but axial length 45 is constrained by the packaging requirements for return spring 50. Too tight clearances between head portion 46 and bore 42 combined with too short axial head length may cause jamming of head portion 46 in bore 42. All of these relationships are known in the RFF prior art and need not to be further elaborated here.

Referring now to FIG. 2, a two-step finger follower rocker arm assembly 100 in accordance with a first embodiment of the invention includes a rocker arm 118 having a body 126 that extends from a first end 128 to a second end 130. A socket 132 integrated in body 126 and positioned proximate to first end 128 receives the head of a hydraulic lash adjuster (HLA) (not shown) for pivotably mounting rocker arm assembly 100 in an engine. Socket 132 may receive any variable oil pressure source. A latching mechanism 140 is provided for engaging and locking a cam-actuated slider member 112, such as high-lift follower 12 shown in FIG. 1, at its most outward extreme of motion.

Latching mechanism 140 includes a stepped bore 142, preferably cylindrical, and integrated in body 126 proximate to the first end 128. Bore 142 is closed by a plug 152 that is secured by a retaining clip 153. Latching mechanism 140 further includes a locking pin 144 that includes a pin head 146 having a first diameter 147 and a pin shank 148 having a smaller second diameter 149. Locking pin 144 is operated as a piston and is axially positioned within bore 142 such that pin head 146 faces plug 152. Plug 152 seals bore 142 and functions as a stop for locking pin 144. Pin shank 148 receives a return spring 150. Locking pin 144 is biased outwards in bore 142 by return spring 150.

A first hydraulic chamber 154 that is in communication via a first passage 156 with socket 132 is formed between plug 152 and pin head 146, and therefore above pin head 146. A second hydraulic chamber 158 that is in communication via a second passage 160 with socket 132 is formed by the cavity housing return spring 150 and is positioned below pin head 146. Body 126 further includes a spray hole 162 that is in communication with first hydraulic chamber 154.

In operation, upon command from an engine control module pressurized oil is supplied via first passage 156 to first hydraulic chamber 154 in known fashion from the hydraulic lash adjuster (HLA, not shown) inserted in socket 132 and simultaneously discharged from second hydraulic chamber 158 via second passage 160. It may further be possible to supply pressurized oil to second hydraulic chamber 158 via second passage 160 and to simultaneously discharge oil from first hydraulic chamber 154 via first passage 156. In any case, first hydraulic chamber 154 and second hydraulic chamber 158 contain oil at substantially the same pressure. The oil pressure in first hydraulic chamber 154 and second hydraulic chamber 158 causes locking pin 144 to become hydraulically biased towards cam-actuated slider member 112 due to the force produced by the pressure acting on the cross-sectional area of the pin shank diameter 149. When such biasing occurs and overcomes the counter-bias force of return spring 150, the end of pin shank 148 positioned opposite from pin head 146 is urged axially into a locking engagement with slider member 112. Locking pin 144 is now in extended position (shown in FIG. 3), since a portion of pin shank 148 axially extends bore 142 towards slider member 112. If this operation is applied to RFF 10 shown in FIG. 1, RFF 10 is operated in high-lift mode. Oil may further exit from first hydraulic chamber 154 through spray hole 162 when locking pin 144 is in extended position, since pin head 144 is not blocking spray hole 162 in the extended position.

By designing two-step finger follower rocker arm assembly 100 to include a first passage 156 and a second passage 160, oil is present in first hydraulic chamber 154 positioned above pin head 146 and in second hydraulic chamber 158 positioned below pin head 146 concurrently. The minor diameter of locking pin 144, which is the second diameter 149 of pin shank 148, determines the hydraulic force generated by the oil pressure available to move locking pin 144 to the extended position and the net volume of oil displaced by the motion of locking pin 144. The hydraulic force can be calculated from oil pressure times cross-sectional area of pin shank 148. With the benefit of the counter-force exerted on locking pin 144 by second passage 160, if the area of pin shank 148 is about 40% smaller than the area of pin head 146, the net force exerted by is the oil pressure against return spring 150 would be 40% less than the force that would be exerted against the return spring without the benefit of second passage 160. Thus, a smaller return spring 150 would be needed than return spring 50. Further, if the area of pin shank 148 is about 40% smaller than the area of pin head 146, then 40% less oil is displaced for the same pin travel compared to prior art rocker arm 18 that includes only passage 56, as shown in FIG. 1. Having to displace a smaller net volume of oil to move locking pin 144 into extended position is beneficial for faster switching response of two-step finger follower rocker arm assembly 100, especially during operation with cold oil.

When the engine control module determines, in known fashion from various engine operating parameters, that a retracted position of locking spring 144 and, thus, disengagement from slider member 112, is desired, a reduced oil pressure is supplied to first hydraulic chamber 154 and to second hydraulic chamber 158, allowing return spring 150 to again bias lock pin 144 away from slider member 112. While moving towards plug 152, locking pin 144 pushes oil contained in first hydraulic chamber 154 out through first passage 156 back to the HLA inserted in socket 132. Simultaneously, oil enters second hydraulic chamber 158 through second passage 160. As long as the reduced oil pressure is maintained in first chamber 154 and second chamber 158, latching mechanism 140 remains in retracted position (shown in FIG. 2) disengaged from slider member 112. If this operation is applied to RFF 10 shown in FIG. 1, RFF 10 is operated in low-lift mode.

Since two-step finger follower rocker arm assembly 100, as illustrated in FIG. 2 in accordance with the invention, enables the presence of oil in both the first chamber 154 and the second chamber 158 and, thus, above and below pin head 146, oil leakage through the clearance between bore 142 and pin head 146 is allowed. Consequently, tolerances between bore 142 and pin head 146 can be relaxed and axial length 145 of pin head 144 can be reduced compared to prior art latching mechanism 40 shown in FIG. 1. A shorter axial length 145 of pin head 144 combined with larger allowable clearances between bore 142 and pin head 146 eliminates the concern of jamming of pin head 144 of the prior art latching mechanism 40 (FIG. 1). The clearance between pin shank 148 and bore 142 below return spring 150 and, therefore, below second hydraulic chamber 158 must be controlled to minimize oil leakage from second hydraulic chamber 158. If needed, the land length in this area could be increased. By designing latching mechanism 140 such that oil may enter and exit first chamber 154 through first passage 156 and second chamber 158 through second passage 160 concurrently, the vent hole incorporated in body 26 of prior art rocker arm 18 can be eliminated.

Referring now to FIG. 3, a two-step finger follower rocker arm assembly 200 in accordance with an alternative embodiment of the invention includes an alternative latching mechanism 240 provided for engaging and locking a cam-actuated slider member 112, such as high-lift follower 12 shown in FIG. 1, at its most outward point of motion. Latching mechanism 240 differs from latching mechanism 140 shown in FIG. 2 by eliminating first passage 156 connecting first hydraulic chamber 154 with HLA socket 132 and by modifying locking pin 144 to permit flow between second hydraulic chamber 158 and first hydraulic chamber 154. In a currently preferred embodiment, latching mechanism 240 includes a locking pin 244 that has an internal oil passage, which communicates chamber 158 with chamber 154.

As shown in FIG. 3, locking pin 244 may include an axial hole 202 extending through pin head 246 and intersecting with a cross hole 204 extending through pin shank 148. Alternatively, slots or grooves (not shown) could be implemented on the outside diameter of pin head 246 to provide an oil passage between second hydraulic chamber 158 and first hydraulic chamber 154. Second hydraulic chamber 158 is in communication with socket 132 via a passage 260.

In operation, upon command from an engine control module, pressurized oil is supplied via passage 260 to second hydraulic chamber 158 in known fashion from the hydraulic lash adjuster (HLA, not shown) inserted in socket 132. The pressurized oil entering second hydraulic chamber 158 also flows into first hydraulic chamber 154 through cross hole 204 and axial hole 202 integrated in locking pin 244. The pressurized oil entering first hydraulic chamber 154 and second hydraulic chamber 158 causes locking pin 244 to become hydraulically biased towards cam-actuated slider member 112. When such biasing occurs and overcomes the counter-bias of return spring 150, the end of pin shank 148 positioned opposite from pin head 246 is urged axially into a locking engagement with slider member 112. Locking pin 244 is now in extended position (shown in FIG. 3), since a portion of pin shank 148 axially extends from bore 142 towards slider member 112. If this operation is applied to RFF 10 shown in FIG. 1, RFF 10 is operated in high-lift mode.

When the engine control module determines, in known fashion from various engine operating parameters, that a retracted position of locking spring 244 and, thus, disengagement from slider member 112, is desired, a reduced oil pressure is supplied to second hydraulic chamber 158 and to first chamber 154 through cross hole 204 and axial hole 202, allowing return spring 150 to again bias lock pin 244 away from slider member 112. While moving locking pin 244 towards plug 152, oil contained in first hydraulic chamber 154 flows out of first hydraulic chamber 154 through axial hole 202 and cross hole 204 into second hydraulic chamber 158 and from second hydraulic chamber 158 through passage 260 to socket 132. The oil flowing out of passage 260 into socket 132 is received by a hydraulic lash adjuster (HLA, not shown). As long as the reduced oil pressure is maintained in second chamber 158, latching mechanism 240 remains in retracted position (shown in FIG. 2) disengaged from slider member 112. If this operation is applied to RFF 10 shown in FIG. 1, RFF 10 is operated in low-lift mode.

The hydraulic force generated by the oil pressure and the net volume of oil displacement by motion of locking pin 244 are the same as described above in connection with FIG. 2. Designing two-step finger follower rocker arm assembly 200 without first passage 156 that must be angled to provide connection from socket 132 to first hydraulic chamber 154 lowers manufacturing costs of assembly 200 by avoiding difficult machining operation. Machining operations to create passage 260 are simpler than machining operation to create passage 156, since passage 260 is shorter and only slightly angled if at all due to the location of socket 132 relative to second hydraulic chamber 158.

While the invention has been described as relating to a two-step rocker arm assembly, it is understood that it can relate to a deactivating rocker arm assembly whereby, instead of a lower valve lift, no valve lift is applied. Furthermore, while the invention has been described in connection with a two-step RFF 10, it may be applicable for other cylinder activation operations.

While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims. 

1. A latching mechanism for variably activating an engine valve in an internal combustion engine, comprising: a body including a bore and a variable oil pressure source; a locking pin including a pin head axially disposed in said bore; a first hydraulic chamber formed above said pin head; a second hydraulic chamber formed below said pin head; a first passage connecting said first hydraulic chamber with said variable oil pressure source; and a second passage connecting said second hydraulic chamber with said variable oil pressure source.
 2. The latching mechanism of claim 1, wherein said locking pin further includes a pin shank that extends from said pin head towards said second hydraulic chamber, and wherein said pin shank has a smaller diameter than said pin head.
 3. The latching mechanism of claim 2, further comprising a return spring received by said pin shank, wherein said locking pin is biased in said bore by said return spring.
 4. The latching mechanism of claim 1, further comprising a plug disposed in said bore, wherein said plug seals said bore and restricts movement of said locking pin.
 5. The latching mechanism of claim 1, wherein said variable oil pressure source receives a hydraulic lash adjuster, wherein said first hydraulic chamber receives pressurized oil from said hydraulic lash adjuster via said first passage while said second hydraulic chamber discharges oil via said second passage concurrently, or wherein said second hydraulic chamber receives pressurized oil from said hydraulic lash adjuster via said second passage while said first hydraulic chamber discharges oil via said first passage concurrently.
 6. The latching mechanism of claim 1, wherein a hydraulic force of pressurized oil received by said first hydraulic chamber and said second hydraulic chamber overcomes a counter-bias force of said return spring and biases said locking pin to an extended position, where a portion of said locking pin positioned opposite from said pin head axially extends beyond said bore.
 7. The latching mechanism of claim 6, wherein said hydraulic force axially urges said locking pin into a locking engagement with a slider member activating said engine valve.
 8. The latching mechanism of claim 6, wherein said diameter of said pin shank determines said hydraulic force that overcomes said counter-bias force of said return spring and a net volume of oil displaced by motion of said locking pin.
 9. The latching mechanism of claim 1, wherein oil enters and exits said first hydraulic chamber via said first passage, wherein oil enters and exits said second hydraulic chamber via said second passage, and wherein compressed oil leaks through a clearance between said pin head and said bore.
 10. The latching mechanism of claim 1, wherein a cavity housing a return spring forms said second hydraulic chamber.
 11. A latching mechanism for variably activating an engine valve in an internal combustion engine, comprising: a body including a bore and a variable oil pressure source; a locking pin axially disposed in said bore, said locking pin including a pin head, a pin shank, and an oil passage; a first hydraulic chamber formed above said pin head; a second hydraulic chamber formed below said pin head; and a passage connecting said second hydraulic chamber with said variable oil pressure source; wherein said oil passage of said locking pin connect said second hydraulic chamber with said first hydraulic chamber.
 12. The latching mechanism of claim 11, wherein said oil passage of said locking pin includes an axial hole extending through said pin head and intersecting with a cross hole extending through said pin shank.
 13. The latching mechanism of claim 11, wherein said oil passage of said locking pin includes slots or grooves disposed on an outside diameter of said pin head.
 14. The latching mechanism of claim 11, wherein pressurized oil is supplied to said second hydraulic chamber from said variable oil pressure source via said passage, and wherein said pressurized oil is supplied from said second hydraulic chamber to said first hydraulic chamber via said oil passage of said locking pin.
 15. The latching mechanism of claim 11, wherein oil flows from said first hydraulic chamber into said second hydraulic chamber via said oil passage of said locking pin, and wherein said oil flows from said second hydraulic chamber into said socket via said passage.
 16. The latching mechanism of claim 11, wherein pressurized oil supplied to said second hydraulic chamber via said passage and supplied to said first hydraulic chamber via said oil passage of said locking pin creates a hydraulic force that overcomes the force of a return spring and urges said locking pin axially into a locking engagement with a slider member for activating said engine valve.
 17. A two-step finger follower rocker arm assembly for variably activating an engine valve in an internal combustion engine, comprising: a rocker arm body having a bore; a locking pin axially disposed in said bore; a first hydraulic chamber formed in said bore, wherein oil flows into and out of said first hydraulic chamber via a first passage; and a second hydraulic chamber formed in said bore, wherein oil flows into and out of said second hydraulic chamber via a second passage; wherein pressurized oil supplied to said first hydraulic chamber and said second hydraulic chamber moves said locking pin to an extended position for selectively latching a slider member to said body to provide a first rocker assembly mode having a first valve lift capability; and wherein a return spring moves said locking pin to a retracted position for unlatching said slider member from said body to provide a second rocker assembly mode having a second valve lift capability.
 18. The rocker arm assembly of claim 17, wherein said first passage connects said first hydraulic chamber with a variable oil pressure source.
 19. The rocker arm assembly of claim 18, wherein said variable oil pressure source receives a hydraulic lash adjuster.
 20. The rocker arm assembly of claim 17, wherein said first passage is an oil passage integrated in said locking pin, and wherein said oil passage connects said first hydraulic chamber with said second hydraulic chamber.
 21. The rocker arm assembly of claim 18, wherein said second passage connects said second hydraulic chamber with the variable oil pressure source.
 22. The rocker arm assembly of claim 17, wherein said locking pin includes a pin head and a pin shank having a smaller diameter than said pin head, wherein said pressurized oil generates a hydraulic force, and wherein said diameter of said pin shank determines the hydraulic force needed to move said locking pin to said extended position.
 23. A method for providing a hydraulic force to a latching mechanism of a rocker arm assembly for variably activating an engine valve in an internal combustion engine, comprising the steps of: axially disposing a locking pin having a pin shank and a pin head in a bore of a rocker arm body; supplying pressurized oil via a first passage to a first hydraulic chamber; discharging pressurized oil via a second passage from a second hydraulic chamber, and determining a hydraulic force needed to overcome a counter force and to move said locking pin to an position where a portion of said pin shank extends from said bore based on a diameter of said pin shank, wherein said diameter of said pin shank is smaller than the diameter of said pin head.
 24. The method of claim 23, further including the steps of: integrating said first passage into said locking pin to form an internal passage; connecting said second hydraulic chamber with a variable oil pressure source via said second passage; and providing communication between said second hydraulic chamber and said first hydraulic chamber via said locking pin oil passage.
 25. The method of claim 23, further including the steps of: connecting said first hydraulic chamber with a variable oil pressure source via said first passage, and connecting said second hydraulic chamber with said variable oil pressure source via said second passage. 